Engine braking systems have been known for many years. Such systems may be particularly useful in heavy vehicles, such as trucks and buses, because these vehicles have heightened braking needs and commonly use diesel engines. Engine braking systems are needed in diesel engine vehicles because of the inherent cylinder aspiration that results from the valve timings (main intake and main exhaust events) that are required for positive power operation.
Past engine braking systems have added compression-release openings of the exhaust valve near the end of the compression stroke to the positive power valve events (i.e., main exhaust events) to affect a braking force on the drive train. During compression-release braking, fuel injection is stopped and the exhaust valves are also opened near the end of the compression stroke to convert a power producing internal combustion engine into a power absorbing air compressor.
Each compression stroke may be used to slow a vehicle equipped with a compression-release brake. During the compression stroke, the piston travels upward and compresses the gases trapped in the cylinder. The compressed gases oppose the upward motion of the piston. During engine braking operation, as the piston approaches top dead center (TDC), the exhaust valves are opened to release the compressed gases to the exhaust manifold, preventing the energy stored in the compressed gases from being returned to the engine on the subsequent expansion down-stroke. In doing so, the engine develops retarding power to help slow the vehicle down. An example of a known compression-release engine brake is provided by the disclosure of the Cummins, U.S. Pat. No. 3,220,392 (November 1965), which is incorporated herein by reference.
Bleeder type engine brakes provide an alternative to compression-release type engines brakes. Known bleeder brakes have added a small amount of lift (x)to the entire exhaust valve opening profile, as shown by the change from exhaust valve lift profile A to profile B in FIG. 1. Thus, known bleeder brakes hold the exhaust valve(s) slightly open during the intake, compression and expansion strokes, and produce an exaggerated main exhaust lift during the exhaust stroke. This is referred to as full-cycle bleeder braking and is illustrated by profile B in FIG. 1. Partial-cycle bleeder braking is also possible. Partial-cycle bleeder braking results when the exhaust valve(s) are maintained slightly open during much, but not all, of the intake, compression and expansion strokes. Typically, a partial-cycle bleeder brake differs from a full-cycle bleeder brake by closing the exhaust valve(s) during most of the intake stroke. An example of a known bleeder type engine brake is provided by the disclosure of Yang, U.S. Pat. No. 6,594,996 (Jul. 22, 2003), which is incorporated herein by reference.
Usually, the initial opening of the braking valve(s) in a bleeder braking operation is far in advance of the compression TDC (i.e., early valve actuation) and then lift is held constant for a period of time. As such, a bleeder type engine brake requires much lower force to actuate the valve(s) due to early valve actuation, and generates less noise due to continuous bleeding instead of the rapid blow-down of a compression-release type brake. Moreover, bleeder brakes often require fewer components and can be manufactured at lower cost. Thus, an engine bleeder brake can have significant advantages.
Despite these advantages, however, bleeder type engine brakes have not been widely used because they typically produce less braking power than the compression-release type brakes. One factor that detracts from the braking power of bleeder brakes is their inability to carry out bleeder braking throughout the entire engine cycle. Previous bleeder brakes have not held the exhaust valve open throughout the engine cycle at a relatively constant lift. Instead, the normal main exhaust valve event (during the exhaust stroke) has been superimposed over the bleeder brake opening, thereby resulting in an exhaust valve lift profile shown as profile B in FIG. 1.
The exhaust valve lift profile B in FIG. 1 not only includes a main exhaust event, but even worse, an exaggerated main exhaust event. The main exhaust event included in profile B has the lift of a normal main exhaust event (profile A), plus the bleeder brake lift (x). This exaggerated lift can affect bleeder braking power negatively. Furthermore, this exaggerated lift can cause the exhaust valve to extend so far into the engine cylinder that valve to piston contact is possible. The risk of valve to piston contact may require that pockets be drilled into the piston to accommodate the exhaust valve. Such pockets can have negative effects on positive power and emissions.
Thus, the present Applicants have determined that the inclusion of the main exhaust event in a bleeder braking cycle may reduce the effectiveness of the bleeder brake and/or reduce the desirability of an engine equipped to provide bleeder braking. Applicants have also determined that the elimination, reduction, or delay of a main exhaust event may impact engine braking positively. Both bleeder braking and compression-release braking may be carried out on a two-cycle basis (i.e., for each up-down stroke of the piston) when the main exhaust event is eliminated, reduced or delayed. Accordingly, there is a need for a bleeder braking system and method that may not include a full main exhaust valve event during bleeder brake or compression-release brake operation.
The braking power of an engine (bleeder and compression-release) brake may be a function of the exhaust back pressure against which the cylinders act. This exhaust back pressure can be regulated in various ways. Three primary ways are through the use of a variable geometry turbocharger (VGT), exhaust gas recirculation (EGR), and exhaust pressure regulation (EPR). Each of these ways of increasing and regulating exhaust pressure may be used singly or in combination to improve engine braking.
VGT's may enable intake and/or exhaust manifold pressures to be increased as compared with those produced using conventional fixed geometry turbochargers. These increased pressures may correspond to improved engine brake performance, especially at low and moderate engine speeds. Although it is recognized that the operation of an engine brake (particularly a bleeder brake) may be preferred when used in conjunction with a VGT, it is recognized that effective engine braking may still be carried out with a fixed geometry turbocharger (FGT).
EGR involves the recirculation of gas from the exhaust manifold side of an engine back to the intake side or to the cylinder of the engine. EGR may be carried out in an engine during positive power and/or engine braking for a number of reasons. For the purposes of this discussion, Applicant's reference to “EGR” is intended to be expansive and includes, but is not limited to, “brake gas recirculation” (BGR) which may be carried out to improve engine braking.
The recirculation of exhaust gas can be carried out in one of two ways. In a first way, referred to as internal EGR, exhaust gas is forced back from the exhaust manifold into the cylinder and potentially further back past the intake valve and into the intake manifold. In the second way, referred to as external EGR, the exhaust manifold gas may be routed through a passage provided between the exhaust manifold and the intake manifold and/or any engine components provided between the two manifolds. Certain performance and emissions advantages may be realized during positive power by using EGR. The affect of EGR on exhaust manifold pressure also may be used during engine braking to control and/or improve braking power because braking power may be a function of exhaust back pressure.
EPR can be achieved by devices designed to restrict the flow of exhaust gas out of the engine. One prime example of such a device is an exhaust brake. An exhaust brake can be created by placing a gate valve, or some other type of restrictive device, in the exhaust system between the exhaust manifold and the end of the tail pipe. When the gate valve is fully or partially closed it increases the exhaust back pressure experienced by the engine. Because the exhaust brake can be selectively actuated, it can provide EPR that is used to modulate engine braking. If the exhaust brake is able to provide selective levels of actuation, it can provide even more sophisticated EPR, and thus improved engine braking control.
The use of VGT's, EGR, and/or EPR may permit the levels of pressure and temperature in the exhaust manifold and engine cylinders to be controlled and maintained such that optimal degrees of engine braking are attained at any engine speed. While it is understood that the inclusion of VGT, EGR, and/or EPR may provide improved engine braking, their inclusion is not required to experience improved braking through the reduction or elimination of the main exhaust valve event from the engine braking cycle. It is therefore an advantage of some, but not necessarily all, embodiments of the present invention to provide methods and systems for achieving engine braking that include the reduction, delay, and/or elimination of the main exhaust valve event during engine braking. Additional advantages of various embodiments of the invention are set forth, in part, in the description that follows and, in part, will be apparent to one of ordinary skill in the art from the description and/or from the practice of the invention.